SPIN DENSITY WAVE ORDER AND FLUCTUATIONS IN (TMTSF)2PF6 AT VERY HIGH MAGNETIC FIELDS

We report extremely high field and frequency NMR measurements of the proton spin-lattice relaxation rate (1/T1) in (TMTSF)2PF6. They probe the spin density wave (SDW) fluctuations in the critical regime near the SDW transition and in the ordered phase at lower temperatures. The highest field measurements were made using the NHMFL hybrid magnet at 44.7 T and a proton NMR frequency of 1.9 GHz. A quadratic increase in the transition temperature is observed that agrees with transport measurements made at lower fields. As the field is increased above 40 T, a change in the SDW critical dynamics is seen. Also, the behavior of 1/T1 vs. the alignment of the magnetic field suggests a magnetic transition at moderate to high fields with the characteristics of a sharp change in the orientation of the SDW polarization, but with parameters that differ from the spin-flop transition seen at low field.